1. Field of the Invention
The present invention relates to an apparatus for varying a capacity in a scroll type compressor having a stationary scroll and a movable scroll opposed to the stationary scroll and rotatable about the stationary scroll but not about its own axis, to reduce the capacity of a closed space defined between the stationary scroll and the movable scroll.
2. Description of the Related Art
In a conventional scroll type compressor as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 61-291792, a volume-reducing area of a closed space connected to the start end of a scroll portion provided at the base end of the stationary scroll is connected to a suction pressure area by a by-pass passage running from a rear side of the base end of the stationary scroll. The by-pass passage has a by-pass opening and closing mechanism provided therein to control the opening and closing of the by-pass passage in association with a coolant pressure. The by-pass opening and closing mechanism is provided with a piston which opens and closes the by-pass passage, and an electromagnetic valve which controls the introduction of the discharge coolant (gas) into a cylinder chamber in which the piston is housed. When the electromagnetic valve is in an open position, the discharge coolant (gas) flows into the cylinder chamber, so that the piston is urged to assume a closed position, by which the by-pass passage is closed, against a return spring.
When the electromagnetic valve is in the closed position, however, no discharge coolant enters the cylinder chamber, and thus the piston is brought to the open position, in which the by-pass passage is opened, by the return spring. Consequently, when the electromagnetic valve is closed, the coolant under compression is returned to the suction pressure area, to reduce the discharge capacity.
In such a known scroll type compressor, however, when the compressor rotates at a high speed, since the closed space which is being reduced in volume instantaneously passes through an inlet of the by-pass passage, often no coolant is returned to the suction pressure area through the inlet of the by-pass passage, in comparison with a smaller number of revolutions of the compressor. To solve the problem, the inlet of the by-pass passage can be made larger, to effectively increase the capacity variability of the compressor especially at a large number of revolutions thereof, but an enlargement of the inlet of the by-pass passage increases the quantity of the coolant returned to the suction pressure area therethrough, thus resulting in an excess variation of the capacity at a low rotational speed. On the contrary, if the inlet is designed to optimize the capacity variation at the low rotational speed of the compressor, a sufficient capacity variability cannot be obtained at a high rotational speed.
Japanese Unexamined Patent Publication (Kokai) No. 62-46164 discloses a suction restricting mechanism which controls the flow rate of the suction coolant and a by-pass opening and closing mechanism associated therewith. In this suction restricting mechanism, the restriction is controlled by the pressure of the coolant before the restriction acts directly on a restricting valve. The control of the by-pass passage opening and closing mechanism is effected by the pressure difference between the suction pressures before and after the suction restricting mechanism, so that the closing of the by-pass passage is effected by a rotary valve which constitutes the by-pass passage opening and closing mechanism. The rotary valve is connected to a piston, and a pressure difference of the suction pressures on the opposite sides of the piston before and after the suction restricting mechanism causes the piston to move to rotate the rotary valve. Namely, when the introduction of the suction coolant is restricted, the by-pass passage is opened. Conversely, when a large flow rate of the suction coolant occurs, the by-pass passage is closed. Accordingly, the associated use of the suction restricting mechanism and the by-pass opening and closing mechanism makes it possible to widen the rotational speed range of the compressor in which a highly effective capacity variability can be obtained.
Nevertheless, a precise control of the closure and opening of the by-pass passage and the restriction cannot be achieved by such a direct control mechanism in which the rotation of the rotary valve is directly controlled by opposing suction pressures before and after the suction restricting mechanism, i.e., the suction pressure prior to restriction in which no cooling load is reflected and the suction pressure after the restriction, and such a direct control mechanism of the restriction in which the restriction is controlled by a direct action of the suction pressure on which the cooling load is reflected onto the restricting valve. In particular, it is very difficult to obtain an optimum capacity variability within a total range of from the low rotational speed to the high rotational speed.